This application is based on and incorporates herein by reference Japanese Patent Application Nos. 2000-308001 filed on Oct. 3, 2000, 2001-31532 filed on Feb. 7, 2001, 2001-65962 filed on Mar. 9, 2001, 2001-77396 filed on Mar. 19, 2001, and 2001-83964 filed on Mar. 23, 2001.
1. Field of the Invention
The present invention relates to an exhaust emission control system for an internal combustion engine, in which a plurality of catalysts or a plurality of catalyst groups are disposed in series in an exhaust passage.
2. Description of Related Art
In recent years, to increase the capability of reducing hazardous substances in exhaust gas of an engine, two catalysts for exhaust emission control are disposed in series at some midpoint of an exhaust pipe of the engine. According to the method, an air-fuel ratio sensor (or oxygen sensor) is disposed on each of the upstream side of an upstream catalyst and the downstream side of a downstream catalyst. An air-fuel ratio closed loop control is performed by detecting the air-fuel ratio of exhaust gas flowing in the upstream catalyst by the upstream sensor and making the detected air-fuel ratio coincide with a target air-fuel ratio. The air-fuel ratio of the exhaust gas passed through the downstream catalyst is detected by the downstream sensor, and the target air-fuel ratio on the upstream side is corrected so that the air-fuel ratio detected on the downstream side coincides with a predetermined value.
Generally, conversion efficiency of a catalyst varies according to a state of adsorbing hazardous components which are generated in a state where the air-fuel ratio is lean (hereinbelow, called components on the lean side) and hazardous components which are generated in a state where the air-fuel ratio is rich (hereinbelow, called components on the rich side) of the catalyst. At and around the stoichiometric air-fuel ratio, the catalyst reduces both components on the rich side (HC, CO, and the like) and components on the lean side (NOx and the like) in the exhaust gas most efficiently, and the highest catalytic conversion efficiency can be obtained. In the conventional air-fuel ratio feedback system, however, there is a tendency that when the amount of adsorbing the components on the rich side of the upstream catalyst is large, that of the downstream catalyst is also large. When the amount of adsorbing the components on the lean side of the upstream catalyst is large, that of the downstream catalyst is also large. As a result, there is a tendency that the states of both the upstream and downstream catalysts are controlled in the same way. Thus, the exhaust gases cannot be treated by efficiently using the two catalysts. Considering that two catalysts are used, an effect of improving the catalytic conversion efficiency is not so great.
In the above-described system, it is desirable to set the adsorption state of both of the upstream and downstream catalysts to a stoichiometric state as much as possible during the engine operation. However, depending on the driving conditions, in order to save the fuel or to prevent an excessive increase in the engine rotation, there is a case such that the fuel cut is executed. Since oxygen in the air taken in the cylinders is not used for combustion but is exhausted as it is to the exhaust pipe during the fuel cut, the lean-side components (oxygen) in the exhaust gases entering the catalysts largely increase, and the lean-side component adsorption amount of the catalysts largely increases. Thus, JP-A-6-200803 and JP-A-8-193537 disclose the techniques such that when the fuel cut is finished and the fuel injection is restarted, the air-fuel ratio is set temporarily to the rich side to make the lean-side components (oxygen) adsorbed by a catalyst react with the rich-side components (HC, CO, and the like) in the exhaust gases, thereby promptly decreasing the lean-side component adsorption amount of the catalyst.
In each of the two publications, only one catalyst is disposed in the exhaust pipe. It can be considered to apply the technique of JP-A-6-200803 to a system having two catalysts as follows. When the fuel cut is finished and the fuel injection is re-started, the rich-side control for setting the air-fuel ratio temporarily to the rich side by about 5-10% is performed to reduce the lean-side component adsorption amount of the catalysts. By the operation, when the output of an air-fuel ratio sensor (or oxygen sensor) on the downstream side changes to a rich output, the rich-side control is stopped and the program returns to the normal control.
However, as the lean-side component adsorption amount of the catalysts decreases during the rich-side control, the amount of rich-side components necessary to reduce the lean-side components also decreases. If the degree of richness in the air-fuel ratio during the rich-side control is fixed, the setting of the air-fuel ratio to the rich side is insufficient when the lean-side component adsorption amount of the catalysts is large at an initial stage of the rich-side control. On the contrary, as the lean-side component adsorption amount of the catalyst becomes small at the end of the rich-side control, the setting of the air-fuel ratio to the rich side becomes excessive, and a rich-side component exhaust amount to the atmosphere increases.
In order to solve the drawback, in JP-A-8-193537, an oxygen adsorption amount of the catalyst during the rich-side control is estimated and the degree of richness is changed according to the oxygen adsorption amount. However, since the maximum oxygen adsorption amount changes by the change with time of each of the catalysts, it is difficult to estimate the oxygen adsorption amount of each catalyst with high accuracy. It is accordingly difficult to properly change the degree of richness in association with the change in the actual oxygen adsorption amount of each of the catalysts during the rich-side control.
A first object of the present invention is to provide an exhaust emission control system of an internal combustion engine with increased catalytic conversion efficiency, capable of efficiently reducing hazardous components in exhaust gas by efficiently using a plurality of catalysts (or catalyst groups) disposed in series in an exhaust passage.
According to a first aspect of the present invention, in an exhaust emission control system of an internal combustion engine, a state of a catalyst or a catalyst group disposed on the upstream side (hereinbelow, called xe2x80x9cupstream catalystxe2x80x9d) is detected or estimated by upstream catalyst state detecting means, and a state of a catalyst or a catalyst group disposed on the downstream side (hereinbelow, called xe2x80x9cdownstream catalystxe2x80x9d) is detected or estimated by downstream catalyst state detecting means. As shown in FIG. 6, an air-fuel ratio is controlled by air-fuel ratio control means so that one of the states of the upstream and downstream catalysts is that an adsorption amount of hazardous components on the rich side is large and the other one is that an adsorption amount of hazardous components on the lean side is large.
For example, when the adsorption amount of the components on the rich side of the upstream catalyst is large, the conversion efficiency of the components on the lean side (NOx and the like) in exhaust gases of the upstream catalyst is high but the conversion efficiency of the components on the rich side (HC, CO, and the like) of the upstream catalyst is relatively low. Consequently, the amount of the components on the rich side in the exhaust gases flowing from the upstream catalyst becomes relatively large. In this case, it is controlled so that the adsorption amount of the components on the lean side of the downstream catalyst is large. Therefore, the components on the rich side which cannot be reduced by the upstream catalyst can be efficiently reduced by the downstream catalyst in which the adsorption amount of the components on the lean side is large. On the other hand, when the adsorption amount of the components on the lean side of the upstream catalyst is large, the adsorption amount of the components on the lean side in exhaust gases flowing from the upstream catalyst is relatively large. In this case, it is controlled so that the adsorption amount of the components on the rich side of the downstream catalyst is large. Consequently, the components on the lean side which cannot be reduced by the upstream catalyst can be efficiently reduced by the downstream catalyst in which the adsorption amount of the components on the rich side is large. In such a manner, the components on the rich and lean sides in the exhaust gases can be efficiently removed by effectively using both the upstream and downstream catalysts. Thus, the catalytic conversion efficiency can be increased.
It is also possible to detect the air-fuel ratio of exhaust gases flown from the upstream catalyst by a sensor, and control the air-fuel ratio so as to be opposite to the rich/lean side of the components of the large adsorption amount in the exhaust gases of the downstream catalyst. In such a manner, the components on the rich and lean sides in exhaust gases can be efficiently reduced by effectively using both the upstream and downstream catalysts. Thus, the catalytic conversion efficiency can be increased. According to a second aspect of the present invention, an exhaust emission control system of an internal combustion engine includes: a first sensor for detecting an air-fuel ratio or a rich/lean state of exhaust gases entering an upstream catalyst; a second sensor for detecting an air-fuel ratio or a rich/lean state of the exhaust gases flowing from the upstream catalyst; and a third sensor for detecting an air-fuel ratio or a rich/lean state of the exhaust gases flowing from a downstream catalyst. A target air-fuel ratio is set by air-fuel ratio closed loop controlling means on the basis of an output of the second sensor and/or an output of the third sensor, and a control range of the target air-fuel ratio is shifted on the basis of the outputs of the second and third sensors. In such a manner, while detecting the converting states of both the upstream and downstream catalysts, the control range of the target air-fuel ratio can be shifted so as to improve the conversion efficiency of the system as a whole. Thus, the exhaust gases can be efficiently treated by efficiently using both the upstream and downstream catalysts.
Generally, the catalytic conversion efficiency changes according to the adsorbing states of the components on the lean/rich sides of the catalysts. When the adsorbing states of the catalysts are around the stoichiometric ratio, both the components on the rich side (HC, CO, and the like) and the components on the lean side (NOx and the like) can be reduced most efficiently, and the highest catalytic conversion efficiency can be obtained.
It is also possible to switch a control gain of a sub-closed loop control for setting the target air-fuel ratio on the basis of an output of the second sensor and an output of the third sensor. In such a manner, the target air-fuel ratio can be changed with high response by switching the control gain of the second closed loop control in accordance with the state of the upstream catalyst and the state of the downstream catalyst. Thus, the exhaust gases can be efficiently treated by efficiently using both the upstream and downstream catalysts.
According to a third aspect of the present invention, an exhaust emission control system of an internal combustion engine of the invention includes: a first sensor for detecting an air-fuel ratio or a rich/lean state of an exhaust gas entering a catalyst or a catalyst group disposed on the upstream side (hereinbelow, called xe2x80x9cupstream catalystxe2x80x9d); a second sensor for detecting an air-fuel ratio or a rich/lean state of the exhaust gas flowing from the upstream catalyst; and a third sensor for detecting an air-fuel ratio or a rich/lean state of the exhaust gas flowing from a catalyst or a catalyst group disposed on the downstream side (hereinbelow, called xe2x80x9cdownstream catalystxe2x80x9d). In the system, a target output of the second sensor upstream of the downstream catalyst (target air-fuel ratio on the upstream side of the downstream catalyst) is set by downstream-side second closed loop control means on the basis of an output of the third sensor downstream of the downstream catalyst. A target output of the first sensor upstream of the upstream catalyst (target air-fuel ratio on the upstream side of the upstream catalyst) is set by upstream-side second closed loop control means on the basis of a deviation between an output of the second sensor upstream of the downstream catalyst and a target output of the second sensor. By air-fuel ratio closed loop controlling means, an air-fuel ratio is closed loop controlled on the basis of a deviation between an output of the first sensor and a target output of the first sensor. Whether an output of the third sensor downstream of the downstream catalyst is normal or not is determined by sensor diagnosing means. When it is determined by the sensor diagnosing means that the output of the third sensor is not normal, by fail-safe means, an operation of the downstream-side second closed loop control means is inhibited, the target output of the second sensor upstream of the downstream catalyst is set to a learn value or a predetermined set value and, on the basis of a deviation between the output of the second sensor and the target output of the second sensor, the target output of the first sensor upstream of the upstream catalyst is set.
With such a configuration, in the case where the output of the third sensor downstream of the downstream catalyst becomes abnormal, the abnormal output of the third sensor is ignored, and the second closed loop control for setting the target output of the first sensor (target air-fuel ratio on the upstream side of the upstream catalyst) can be performed by using the output of the second sensor upstream of the downstream catalyst which functions normally (air-fuel ratio of exhaust gases flowing from the upstream catalyst). Consequently, even when the output of the third sensor used for the second closed loop control becomes abnormal, the second closed loop control in which the state of the upstream catalyst is reflected can be carried out by using the second sensor which functions normally.
The following manner is also possible. Whether an output of the second sensor upstream of the downstream catalyst is normal or not is determined. When it is determined that the output of the second sensor is not normal, operations of both the upstream-side and downstream-side second closed loop control means are inhibited, and the target output of the first sensor may be set on the basis of an output of the third sensor downstream of the downstream catalyst. With such a configuration, in the case where the output of the second sensor upstream of the downstream catalyst becomes abnormal, the abnormal output of the second sensor is ignored, and the second closed loop control for setting the target output of the first sensor (target air-fuel ratio on the upstream side of the upstream catalyst) can be performed by using the output of the third sensor downstream of the downstream catalyst which functions normally (air-fuel ratio of exhaust gases flowing from the downstream catalyst). Thus, even when the output of the second sensor used for the second closed loop control becomes abnormal, the second closed loop control in which the states of the two catalysts are reflected to some extent can be carried out by using the third sensor which functions normally. Worseness in the exhaust gas conversion efficiency can be minimized.
According to a fourth aspect of the present invention, an exhaust emission control system of an internal combustion engine has: a first sensor for detecting an air-fuel ratio or a rich/lean state of an exhaust gas entering a catalyst or a catalyst group disposed on the upstream side (hereinbelow, called xe2x80x9cupstream catalystxe2x80x9d); a second sensor for detecting an air-fuel ratio or a rich/lean state of the exhaust gas flowing from the upstream catalyst; and a third sensor for detecting an air-fuel ratio or a rich/lean state of the exhaust gas flowing from a catalyst or a catalyst group disposed on the downstream side (hereinbelow, called xe2x80x9cdownstream catalystxe2x80x9d). In the system, by downstream-side second closed loop control means, a target output of the second sensor upstream of the downstream catalyst (target air-fuel ratio on the upstream side of the downstream catalyst) is set on the basis of an output of the third sensor downstream of the downstream catalyst. By upstream-side second closed loop control means, a target output of the first sensor upstream of the upstream catalyst (target air-fuel ratio on the upstream side of the upstream catalyst) is set on the basis of a deviation between an output of the second sensor upstream of the downstream catalyst and a target output of the second sensor. By air-fuel ratio closed loop controlling means, an air-fuel ratio is closed loop controlled, on the basis of a deviation between an output of the first sensor and a target output of the first sensor. By learning means, the target output of the second sensor is corrected by learning on the basis of a deviation between the output of the second sensor and the output of the third sensor.
In such a manner, even when the output characteristic of the second or third sensor is deviated to the lean or rich side due to manufacture variations, deterioration with time, and the like, the deviation is learned, and the target output of the second sensor can be corrected so as to compensate the deviation. Consequently, the high-precision air-fuel ratio control in which the deviation in the control system due to manufacture variations, deterioration with time, and the like of the sensor system is compensated can be executed. The exhaust gas reducing efficiency can be improved without being influenced by the manufacture variations, deterioration with time, and the like of the sensor system.
A second object of the present invention is to provide an exhaust emission control system of an internal combustion engine with improved exhaust gas reducing efficiency, for controlling the air-fuel ratio while detecting the states of upstream and downstream catalysts by three air-fuel ratio sensors (or oxygen sensors), in which when a lean-side component adsorption amount (oxygen adsorption amount) of each of the catalysts becomes excessive as in the time of a fuel cut, the lean-side component adsorption amount of each of the catalysts can be promptly reduced.
According to a fifth embodiment, an exhaust emission control system of an internal combustion engine according to the invention has: a first sensor for detecting an air-fuel ratio or a rich/lean state of an exhaust gas entering a catalyst or a catalyst group disposed on the upstream side (hereinbelow, called xe2x80x9cupstream catalystxe2x80x9d); a second sensor for detecting an air-fuel ratio or a rich/lean state of the exhaust gas flowing from the upstream catalyst; and a third sensor for detecting an air-fuel ratio or a rich/lean state of the exhaust gas flowing from a catalyst or a catalyst group disposed on the downstream side (hereinbelow, called xe2x80x9cdownstream catalystxe2x80x9d). In the system, by air-fuel ratio closed loop controlling means, a target air-fuel ratio is set on the basis of an output of the second sensor and/or an output of the third sensor, and an air-fuel ratio is closed loop controlled on the basis of a deviation between the target air-fuel ratio and an output of the first sensor. When it is estimated that a lean-side component adsorption amount of the upstream catalyst and/or the downstream catalyst is equal to or larger than a predetermined amount due to a fuel cut or the like, by rich-side control means, a rich-side control for setting an air-fuel ratio temporarily to the rich side is executed. During the rich-side control, the degree of richness in the air-fuel ratio is changed on the basis of an output of the second sensor and/or an output of the third sensor. With such a configuration, during the rich-side control, the degree of richness in the air-fuel ratio can be changed in accordance with the lean-side component adsorption amount (oxygen adsorption amount) of each of the upstream and downstream catalysts. Thus, the lean-side component adsorption amount of each of the catalysts is promptly reduced, and the exhaust gas reducing efficiency can be improved.